Showing posts with label water treatment. Show all posts
Showing posts with label water treatment. Show all posts

Wednesday, April 26, 2017

The Ten Things Everyone Should Know about pH and ORP

Reprinted with permission from AquaMetrix Instruments

Here is a list of the ten things anyone in the business of measuring the pH or ORP of their process should know that will make his or her job more stress-free.

1. pH measurements are only good to 0.1 pH units.

Electrodes are funny things. They are the only electronic components that don’t even have specifications listed in their data sheets. One major figure of merit, the impedance of the glass electrode, is on the order of megaohms and can vary by a factor of two. Cross sensitivity to other ions (e.g. sodium), response time and differences between any two electrodes limit the accuracy of measurement. Expecting ac- curacy of greater than 0.1 pH units is

2. Speaking of accuracy... It is not the same as precision.

For a consistent process a pH probe can achieve precision of results to within 0.02 units but it’s accuracy will always be limited by variables such as calibration accuracy, high sodium content or careful routine calibration, however, will narrow the gap between the accuracy of readings closer to the lower level of precision.

3. ORP measurements are only good to ± 20 mV.

Once again the measurement of ORP might be characterized by a high precision but the accuracy of the reading is constrained by the dif culty of calibration, as explained in point 6, and the non-buffered calibration solutions that allow the ORP value of the calibration solutions to change over time. Whereas the buffered composition of pH calibration solutions insures that they will change minimally an ORP calibrations solution is a mixture of Fe2+ and Fe3+ salts. Just the addition of air to the mixture will increase the ORP of the mixture. So don’t look for “NIST traceable” on the label of an ORP calibration solution.

4. ORP measurements are relative.

The process electrode is nothing more than a platinum (or gold) band upon which oxidation (reduction) reactions take place. To complete the circuit, as in all potentiometric devices, is a
reference electrode. Usually that is the same Ag/AgCl electrode used in a pH probe so the REDOX potential that you read is the difference between the Pt band process electrode and the arbitrarily chosen reference electrode. What matters most with an ORP measurement is its change to an agreed upon standard.

5. pH calibration requires two points.

Calibration measures the response of an instrument as one changes the measurement variable in a known way. For pH measurements that measurement variable is the concentration of hydrogen ions. One calibrates a pH probe by drawing a line through points representing the response of a pH probe to more than one H+ ion concentrations (or pH values). Therefore calibration requires at least two points.

6. ORP calibration can only realistically be done with one point.

This sounds like a reversal of point 4 but it’s not. ORP is not a measure of any one species (e.g. H+ ions or oxygen molecules). It measures the collective REDOX potential of everything in the water. Furthermore calibration solutions, e.g. 200 mV Light’s solution and 600 mV Zobell’s solution are two completely different mixtures of reagents. Therefore all we can is choose one calibration solution and calibrate for it.

7. ORP measurements can be slow.

Stick an ORP probe in a calibration solution and you will get a steady reading with- in half a minute. Take the same probe and stick it in a glass of tap water and it might take 20 minutes for the reading to settle to the 200-300 mV that is typical of tap water. The response of the process electrodes to the REDOX reactions that take place on the surface of a Pt electrode depends on the speed of the many reactions that give the potential and the rate at which molecules diffuse through the water. The Fe2+ and Fe 3+ ions that comprise most of the ORP value in calibration solutions react very quickly with the Pt but the Cl- and dissolved oxygen that make up tap water react much more slowly. So the key to successful ORP measurement is patience.

8. pH measurements must be temperature compensated to be accurate.


A pH measurement is the determination of H+ ions in solution. Higher temperature causes the chemical activity to increase and the pH reading to increase accordingly. So we must remove the temperature effect by measuring it and using the well known Nernst equation to correct it for the reading at 250C. (The correction is quite simple. The pH value is proportional to temperature when the latter is an absolute value (i.e. in Kelvins).

9. ORP measurements are affected by temperature but are NOT corrected for it.

An ORP value simply reflects the ability of whatever is in the water to oxidize whatever contaminants are in the water. Of course oxidation speeds up at higher temperatures. But since ORP measures the rate of chemical reactions and not any one chemical species there is no need to correct it. However we can convert the temperature reading to the ORP that we would measure at 250 C so that we have a basis for comparing the chemistry of the process. That’s why we provide a temperature sensing thermistor or RTD with our differential ORP probes.

10. A differential probe properly cared for will last a long time but it won’t last forever.

Over time chemicals in the process make their way through the junction or salt bridge and into the pH 7 buffer that bathes the reference electrode. Manufacturers go to great length to minimize this contamination but they can only slow it down. Aquametrix differential probes allow the user to cheaply and quickly replenish both the pH 7 solution and the salt bridge so that our probes our industry leaders when it comes to probe lifetime. Nonetheless electrodes themselves lose their efficiency as the glass becomes contaminated and/or eroded by the process. However the good news that, with routine calibration and maintenance an Aquametrix differential probe can last for years in most environments. As the car ads say, “your mileage will vary” but rest assured there is no probe on the market that will outlast an Aquametrix differential probe... as long as you take good care of it.

Monday, March 21, 2016

The Ten Things Everyone Should Know about pH and ORP

pH ORP probe
pH ORP probe
(courtesy of AquaMetrix)
Reprinted with permission from AquaMetrix

1. pH measurements are only good to 0.1 pH units

Electrodes are funny things. They are the only electronic components that don’t even have specifications listed in their data sheets. One major figure of merit, the impedance of the glass electrode, is on the order of megahoms and can vary by a factor of two. Cross sensitivity to other ions (e.g. sodium), response time and differences between any two electrodes limit the accuracy of measurement. Expecting accuracy of greater than 0.1 pH units is unrealistic.

2. Speaking of accuracy... It is not the same as precision.

For a consistent process a pH probe can achieve precision of results to within 0.02 units but it’s accuracy will always be limited by variables such as calibration accuracy, high sodium content or Careful routine calibration, however, will narrow the gap between the accuracy of readings closer to the lower level of precision.

3. ORP measurements are only good to ± 20 mV. 

Once again the measurement of ORP might be characterized by a high precision but the accuracy of the reading is constrained by the difficulty of calibration, as explained in point 6, and the non-buffered calibration solutions that allow the ORP value of the calibration solutions to change over time. Whereas the buffered composition of pH calibration solutions insures that they will change minimally an ORP calibrations solution is a mixture of Fe2+ and Fe3+ salts. Just the addition of air to the mixture will increase the ORP of the mixture. So don’t look for “NIST traceable” on the label of an ORP calibration solution.

4. ORP measurements are relative.


The process electrode is nothing more than a platinum (or gold) band upon which oxidation (reduction) reactions take place. To complete the circuit, as in all potentiometric devices, is a reference electrode. Usually that is the same Ag/AgCl electrode used in a pH probe so the REDOX potential that you read is the difference between the Pt band process electrode and the arbitrarily chosen reference electrode. What matters most with an ORP measurement is its change to an agreed upon standard.

5. pH calibration requires two points.


Calibration measures the response of an instrument as one changes the measurement variable in a known way. For pH measurements that measurement variable is the concentration of hydrogen ions. One calibrates a pH probe by drawing a line through points representing the response of a pH probe to more than one H+ ion concentrations (or pH values). Therefore calibration requires at least two points.

6. ORP calibration can only realistically be done with one point.

This sounds like a reversal of point 4 but it’s not. ORP is not a measure of any one species (e.g. H+ ions or oxygen molecules). It measures the collective REDOX potential of everything in the water. Furthermore calibration solutions, e.g. 200 mV Light’s solution and 600 mV Zobell’s solution are two completely different mixtures of reagents. Therefore all we can is choose one calibration solution and calibrate for it.

7. ORP measurements can be slow.

Stick an ORP probe in a calibration solution and you will get a steady reading with- in half a minute. Take the same probe and stick it in a glass of tap water and it might take 20 minutes for the read-

ing to settle to the 200-300 mV that is typical of tap water. The response of the process electrodes to the REDOX reactions that take place on the surface of a Pt electrode depends on the speed of the many reactions that give the potential and the rate at which molecules diffuse through the water. The Fe2+ and Fe 3+ ions that comprise most of the ORP value in calibration solutions react very quickly with the Pt but the Cl- and dissolved oxygen that make up tap water react much more slowly. So the key to successful ORP measurement is patience.

8. pH measurements must be temperature compensated to be accurate.


A pH measurement is the determination of H+ ions in solution. Higher temperature causes the chemical activity to increase and the pH reading to increase accordingly. So we must remove the temperature effect by measuring it and using the well known Nernst equation to correct it for the reading at 250C. (The correction is quite simple. The pH value is proportional to temperature when the latter is an absolute value (i.e. in Kelvins).

9. ORP measurements are affected by temperature but are NOT corrected for it.

An ORP value simply reflects the ability of whatever is in the water to oxidize whatever contaminants are in the water. Of course oxidation speeds up at higher temperatures. But since ORP measures the rate of chemical reactions and not any one chemical species there is no need to correct it. However we can convert the temperature reading to the ORP that we would measure at 250 C so that we have a basis for comparing the chemistry of the process. That’s why we provide a temperature sensing thermistor or RTD with our differential ORP probes.

10. A differential probe properly cared for will last a long time but it won’t last forever.

Over time chemicals in the process make their way through the junction or salt bridge and into the pH 7 buffer that bathes the reference electrode. Manufacturers go to great length to minimize this contamination but they can only slow it down. Aquametrix differential probes allow the user to cheaply and quickly replenish both the pH 7 solution and the salt bridge so that our probes our industry leaders when it comes to probe lifetime. Nonetheless electrodes themselves lose their efficiency as the glass becomes contaminated and/or eroded by the process. However the good news that, with routine calibration and maintenance a differential probe can last for years in most environments.

Tuesday, October 20, 2015

Township Water Authority Uses Ultrasonic Clamp-On Flowmeters to Avoid Surcharges for Exceeding Peak Limits

Ultrasonic Clamp-On Flowmeter
Ultrasonic Clamp-On Flowmeter
(courtesy of Siemens)
Reprinted with permission from Siemens Process Instrumentation

A suburban township buys their drinking water from a major municipal water district. The township’s water distribution system network has four connections to the larger municipality’s water transmission main. The municipality has many customers and has implemented contracts with each of its wholesale customers that limit the peak flows and the time of day in which they may occur. If the wholesale customer exceeds the limit, they are assessed significant surcharges.

Because of the potential surcharges, the wholesale customers can financially justify investing in solutions to better control their water demand, minimize the usage peaks, and control what time of day they occur. These measures include elevated water storage towers, as well as control valves at each of the connections to the municipal provider’s transmission main.

Challenge

The major municipal water district owns and operates “metering pits” with magmeters immediately upstream of the control vaults owned by each customer. However, as a rule, the signals from these meters are not made available to the any wholesale customers on a real time basis. Wholesale water customers are only given datalog summary reports from these meters on a routine schedule for billing purposes.

Without a method of measuring flows or getting flowrate data from the water district in advance, the township customer had no means of knowing, in real-time, the amount of flow they drew from the transmission main. Therefore, they did not know if or when they were exceeding the contractual peak flowrate limits and incurring significant surcharges from the water district until they were billed.

The township customer needs to know the flowrate at each of its four connections to the transmission main so they may control how much is being drawn at each site. They also need the total flow from the municipality’s transmission main, so they do not exceed their contractual peak demand.

The control vaults were initially installed without flowmeters. The intention was to use control valve position and upstream/ downstream (differential) pressure readings to estimate the flow through the control valve using the characteristic curve of the valve. This proved to be too complicated and cumbersome for their SCADA system to effectively implement.

Solution

The local Siemens representative worked with the township and their engineer to find a solution to measure the flow rate and totalize the volume of flow at each of the customer’s control vault sites. The most significant challenge was the piping configuration. All of the vaults were previously constructed without provisions for a flowmeter. The control valve vaults are very tight. The Siemens representative used a Siemens ultrasonic clamp-on flowmeter demo kit to demonstrate the technology to the customer, and prove that it would reliably meet their objectives. The vault with the worst piping configuration was selected for the demonstration. That would demonstrate that if the flowmeter would work in the worst site, it would work at the other three sites as well. However, if the Siemens flowmeter didn’t work in that site, the township would need to look at alternate, more costly, flow measurement technology for a solution. Within minutes of arriving on site, the unit was installed and providing reliable readings. The unit was allowed to log for a period of three days. After that, it was retrieved and compared to the readings from a competitive magmeter in the municipal water provider’s metering pit.

The logger on the Siemens clamp-on flowmeter provides helpful information on the quality of the velocity and flow measurements. This logged information helped establish and solidify the confidence of the owner and the engineer that the Siemens clamp-on meter would work for these applications.

Four key reasons the customer chose Siemens flowmeters:
  • The Siemens clamp-on flowmeter has the capability to make the tough measurements and provide information on the quality of those measurements. 
  • The attentive, professional and knowledgeable service they received from the local Siemens representative was well supported by Siemens personnel. 
  • The local representative provided the field service to install the transducers on the pipe, and commission the transmitters. 
  • The local representative conducted the demonstration and assisted the township engineer with their evaluation of the ultrasonic clamp-on flowmeter vs. magmeters owned by the water district. The major water district supported the Siemens ultrasonic clamp-on technology used by the township customer after they attended a Siemens Level & Flow Seminar held in their district. 
Benefits
  • Cost Savings - If they were not able to use the Siemens ultrasonic clamp-on flowmeters, the customer would have had to excavate and install a below-grade vault to house a magmeter and associated isolation and by-pass valves, along with conduit and wiring, at each of these four sites. This would have required cutting the water pipe and then going through a cumbersome disinfection process, both of which would have required lengthy permitting and costly testing. Further, some of the sites really had little or no room to accommodate such a structure or piping modifications. It is estimated these modifications would have totaled over $250,000. In comparison, the customer ended up spending $25,000 for the meters, and field service to install some conduit from the pipe to an existing above grade SCADA panel. 
  • Time Savings - The customer had already made improvements to the distribution system and installed four new control vaults. Their construction contracts were closing and they could not use their water tower until the new flow controls were added. Time was a critical factor. The customer saved 3-6 months in time by using the Siemens clamp-on flowmeters instead of having to construct new vaults to house magmeters. 
  • Improved Process Reliability - Now that the meters are in place, the customer can control how much water they are taking from the water district at each of these four locations, and ensure they do not exceed their contractual peak. They can now also properly manage the fill and draw of their elevated storage tank to offset peak demands, and fill/store during periods of low demand.